Assisted landing for manned aircraft or drones is achieved in a continuous manner using transponder beacons. The aircraft is equipped with at least one beacon, the latter re-transmitting received signals, the received signals originating from the transmission of a ground radar system. The echoes retransmitted by the beacon enable notably the radar system to compute the position of the beacon and therefore of the aircraft.
The beacons used are bistatic transponder beacons usually embodied with a frequency transposition making it possible to separate reception from transmission. Current solutions have several drawbacks.
They require a complicated and costly electronic system on board the targets, on board aircraft in particular. Such a system weighs a good deal, consumes large amounts of energy and offers relatively poor reliability.
Moreover, the occupied frequency band is wide whereas frequencies are becoming harder and harder to allocate. Filtering problems during use of this type of wideband solution are not easy to solve when the use of frequency agility is desired. Moreover, this solution is sensitive to vibrations because the quartz used to precisely generate the transposition frequency mixed with the received frequency is in the critical channel of the frequency retransmitted by the beacon.
Assisted landing systems, incorporating the beacons equipping aircraft must be precise and reliable, notably when landing is difficult and requires great precision, for example in the case of deck-landing. The deck-landing of a helicopter, manned or unmanned, may be considered by way of example. Assisted deck-landing is therefore carried out using transponder beacons equipping the helicopters. In the final phase of deck-landing, the helicopter moves at substantially the same velocity as the ship on which it must touch down. Its velocity is therefore quasi-nil with respect to this ship and therefore with respect to the radar system placed on this ship. Moreover, it is difficult to rely on the blade velocity, specifically because the blade velocity is inappropriate for precise measurements and contributes noise to the measurements. In particular, where the measurement of the blade flashes is concerned, the angular separation linked to “glint” effects is too large in relation to the precision of location required.
For this assisted landing radar system, the velocity of the target, the transponder beacon, is nil. The situation is therefore that of the detection of a fixed target. The echo produced by the beacon can then be difficult to intercept because it is drowned in the ambient noise. Increasing detection performance then necessitates an increase in the complexity of current systems such as for example the system described previously.